Timeline Description: The Manhattan Project is the name given to the U.S. atomic weapons program during World War II. This program produced the first atomic weapons, eventually leading to the bombings of Hiroshima and Nagasaki in August 1945.
| Date | Event |
|---|---|
| August 2, 1939 | Einstein Signed the Einstein-Szilard Letter On August 2, 1939, Albert Einstein signed the Einstein-Szilard Letter. This letter encouraged Roosevelt to actively begin work on an atomic weapon to potentially take action against the growing power of Nazi Germany. |
| September 3, 1939 | World War II Began in Europe World War II began with the Nazi invasion of Poland in 1939. Britain provided an ultimatum, and when the time ticked down on that ultimatum, war began. |
| October 11, 1939 | Einstein-Szilard Letter Delivered to Roosevelt The Einstein-Szilard letter was delivered to Roosevelt by economist Albert Sachs. Roosevelt immediately established a Uranium Committee to begin research. |
| October 21, 1939 | First Meeting of Uranium Committee Lyman Briggs handled the first meeting of the Uranium Committee. The Committee was given a $6,000 budget to begin their work. |
| March 2, 1940 | Neil Bohr's Hypothesis Verified Neil Bohr's hypothesis that Uranium 235 is responsible for fission is verified at Columbia University. |
| June 12, 1940 | National Research Defense Committee Created In June 1940, the National Research Defense Committee was created, absorbing the Uranium Research Committee. The budget was increased to $40,000. |
| February 25, 1941 | Discovery of Plutonium Plutonium was conclusively discovered on February 25, 1941 by Glenn Seaborg and Arthur Wahl. |
| June 28, 1941 | Office of Scientific Research and Development Created The Office of Scientific Research and Development was created on June 28, 1941. The OSRD absorbed the National Research Defense Committee. |
| December 7, 1941 | Attack on Pearl Harbor On December 7, 1941, the Japanese attacked the U.S. naval base at Pearl Harbor, Hawaii. This forced the immediate entry of the United States into World War II. |
| January 19, 1942 | Roosevelt Authorized Atomic Bomb Project In January 1942, Franklin D. Roosevelt authorized the Manhattan Project to develop atomic weapons to use against the enemies of the United States. |
| September 29, 1942 | Acquisition of Site X The U.S. government acquired land in Oak Ridge, Tennessee as the first research site for the Manhattan Project. This was called Site X. |
| November 16, 1942 | Los Alamos Declared Site Y Los Alamos, New Mexico was chosen as Site Y for the development of an atomic bomb. |
| April 1, 1943 | Los Alamos Laboratory Established The research laboratory at Los Alamos was established in April 1943. This research laboratory would be fully responsible for the development of the atomic bomb. |
| January 11, 1944 | Special Group Appointed to Study Implosion at Los Alamos On January 11, 1944, a special group was appointed to study the possibilities of implosion rather than explosion. |
| July 4, 1944 | Thin Man Abandoned, Fat Boy Prioritized The explosion-based Thin Man project was fully abandoned in July 1944. Fat Boy, an implosion based weapon, and Little Boy, a gun-based weapon, remained in development. |
| May 7, 1945 | Surrender of Nazi Germany Nazi Germany surrendered on May 7, 1945, or Victory Europe day. The war in the Pacific raged on. |
| July 16, 1945 | Test of the Gadget On July 16, 1945, the U.S. conducted the first test of an atomic weapon, called the Gadget. |
| July 25, 1945 | Atomic Weapon Use Authorized The United States authorized the use of atomic weapons against key targets in Japan, including Hiroshima and Nagasaki on July 25. The following day, the U.S. provided Japan with a final warning about devastating damage. |
| August 6, 1945 | Little Boy Deployed on Hiroshima On August 6, 1945, Little Boy was deployed by the B-29 bomber Enola Gray. Little Boy was a gun-type Uranium 235 based weapon. |
| August 6, 1945 | Fat Man Deployed on Nagasaki As the Enola Gray bombed Hiroshima, another B-29, the Bockscar, dropped a plutonium based implosion bomb on Nagasaki. |
| August 12, 1945 | Surrender of Japan Japan offered its unconditional surrender on August 12, 1945, commonly called Victory Japan day in the United States. |
| January 1, 1947 | Atomic Energy Act of 1946 The Manhattan Project was officially dismantled on January 1, 1947 in response to the Atomic Energy Act of 1946. |